1. Field of the Invention
The present invention relates generally to a communication system, and in particular to a method for controlling a sleep-mode operation in a communication system.
2. Description of the Related Art
Communication systems are being developed to provide high-speed large data transmission/reception services to Mobile Stations (MSs). A primary example of these communication systems is the Institute of Electrical and Electronics (IEEE) 802.16e communication system, in which an MS maintains connection to a Base Station (BS) in normal mode.
The MS continuously monitors the downlink to receive data from the BS. Since the monitoring continues even when either the BS or the MS has no transmission data for the other party, the power consumption of the MS increases.
Considering that the IEEE 802.16e communication system was designed to support mobility to the MS, power consumption of the MS is a significant factor that affects overall system performance. To minimize this power consumption of the MS, a sleep-mode and an awake-mode operation are defined for the MS and the BS. Also, the MS periodically ranges to the BS to compensate for power as well as timing offset and frequency offsets, to thereby cope with changes in the channel status between the MS and the BS.
FIG. 1 illustrates a conventional signal flow for the sleep-mode operation in a communication system.
Referring to FIG. 1, to transition from an awake mode to sleep mode, an MS 100 transmits a MOBile_SLeeP-REQuest (MOB_SLP-REQ) message to a BS 110 in step 101. The BS 110 determines whether to approve the transition to the sleep mode based on the statuses of the BS 110 and the MS 100 and transmits a MOBile_SLeeP-ReSPonse (MOB_SLP-RSP) message corresponding to the determination result to the MS 100 in step 103. The MOB_SLP-RSP message includes a listening interval parameter. If the BS 110 has data to transmit to the MS 100 during a listening interval of the sleep mode, it can transmit to the MS 100 a MOBile_TRaFfic-INDication (MOB_TRF-IND) message with an IDentification (ID) of the MS 100 during the listening interval.
Upon receipt of the MOB_SLP-RSP message from the BS 110, the MS 100 starts a sleep-mode operation in accordance with the MOB_SLP-RSP message. The MS 100 is aware that it is to operate in accordance with the listening interval parameter included in the MOB_SLP-RSP message. Even though the MS 100 is in the sleep mode, if it has data to transmit to the BS 110, it can immediately wake up from the sleep mode immediately.
In step 105, the BS 110 transmits a MOB_TRF-IND message to the MS 100 during a listening interval of the sleep mode. Herein, it is assumed that the MOB_TRF-IND message does not include the ID of the MS 100. Thus, the MS 100 decodes the MOB_TRF-IND message and resumes the sleep-mode operation, confirming the absence of its ID in the message.
Upon generation of transmission data for the MS 100 in a listening interval of the sleep mode some time later, the BS 110 transmits to the MS 100 a MOB_TRF-IND message with the ID of the MS 100 in step 107. After decoding the MOB_TRF-IND message, the MS 100 transitions to the awake mode, confirming the presence of its ID, and receives the data from the BS 110.
After the data transmission/reception, the MS 100 and the BS 110 exchange a MOB_SLP-REQ message and a MOB_SLP-RSP message so that the MS 100 can return to the sleep mode. The message exchange leads to waste of uplink and downlink resources and power consumption. In addition, since the MS 100 has to perform bandwidth ranging by transmitting a BandWidth-REQuest (BW-REQ) message in order to be allocated a bandwidth in which it will transmit the MOB_SLP-REQ message, this increases a time delay in the MS's transitioning to the sleep mode.
The IEEE 802.16e communication system defines the following messages to support the above-described sleep-mode operation and awake-mode operation.
(1) MOB_SLP-REQ
The MS transmits the MOB_SLP-REQ message to the BS when it intends to transition from the awake mode to the sleep mode. The MOB_SLP-REQ message includes parameters, i.e. Information Elements (IEs) required for the transitioning to the sleep mode, as listed in Table 1 below.
TABLE 1SizeSyntax(bits)NotesMOB_SLP-REQ_Message_format( ) { Management message type = 508 Number of Classes8Number of powersaving classes for (i=0; i<Number of Classes; i++) {  Definition1  Operation1  Power_Saving_Class_ID6  if (Operation = 1) {  Start_frame_number6  Reserved2  }  if (Definition = 1) {  Power_Saving_Class_Type2  Direction2  Traffic_triggered_wakening_flag1  reserved3  initial-sleep window6  listening-window8  final-sleep window base10  final-sleep window exponent3  Number_of_Sleep_CIDs3  for (i=0; i<Number_of_Sleep_CIDs;  i++ {   CID16  } } TLV encoded informationvariable}
Referring to Table 1, the MOB_SLP-REQ message has a plurality of IEs. Management Message Type indicates the type of the transmitted message. When Management Message Type is 50, it identifies the MOB_SLP-REQ message. Number of Classes is the number of power saving classes included in the MOB_SLP-REQ message. Definition defines a new power saving class or defines an existing power saving class. Operation indicates whether the power saving class is activated or deactivated. Power_Saving_Class_ID is the ID of the power saving class associated with a current operation. Start_frame_number indicates the activation time of the power saving class. Power_Saving_Class_Type indicates the type of the power saving class. Power saving class types are given as follows.
1) Type 1: a class for which the conventional sleep-mode operation is carried out. The MS transitions to the awake mode when data transmission/reception occurs during a listening interval or a MOB_TRF-IND message with a positive indication is received.
2) Type 2: a sleep window size is fixed and data is transmitted/received during a listening interval. The sleep window alternates with the listening interval.
3) Type 3: compared to Type 1 and Type 2 in which the sleep mode is kept unless a mode transition request message is received, Type 3 is a class that automatically ends the sleep mode after one sleep-mode operation, i.e. one sleep window. Type 3 is used mainly for management messages or multicast traffic.
Direction indicates uplink or downlink. Traffic_Triggered_Wakening_Flag (TTWF) applies only for power saving class Type 1 identified by Power_Saving_Class_Type. To be more specific, TTWF is used when the MS intends to maintain the sleep mode despite generation of data during a listening interval.
That is, if TTWF=0, the MS transmits/receives data during a listening interval and transitions to the sleep mode when the listening interval ends, that is, a sleep window starts. If the BS wants to transmit a Medium Access Control (MAC) Service Data Unit (SDU) for the power saving class during the listening interval, or if the MS transmits a BW-REQ message for a connection of the power saving class, or if the MS receives a MOB_TRF-IND message having a positive indication, i.e. the ID of the MS from the BS, the MS can terminate the sleep mode and transition to the awake mode. Also, the MS can terminate the sleep mode by a transaction with a MOB_SLP-REQ message and a MOB_SLP-RSP message.
When TTWF=1, if the MS receives a Packet Data Unit (PDU) from the BS during a listening interval or a management message commanding termination of the sleep mode, for example, a MOB_SLP-RSP message or a DownLink (DL) Sleep Control Extended Subheader, it should end the sleep mode unconditionally and transition to the awake mode. Also, when data is generated within the MS or when the MS transmits a management message requesting termination of the sleep mode, that is, a MOB_SLP-REQ message or a BW-REQ message and an uplink sleep control header to the BS, the MS should end the sleep mode unconditionally and transition to the awake mode. In other words, if TTWF=1, the MS wakes up to the awake mode, upon generation of traffic or a management message during a listening interval.
As described above, TTWF is used for the MS to maintain the sleep mode and transmit/receive data during a listening interval, for power saving class Type 1 as done with Type 2.
Initial-sleep Window indicates the start of the first sleep window. Listening Window indicates a requested listening interval. The maximum value of the sleep window depends on two parameters, final-sleep window base and final-sleep window exponent. The maximum sleep window is (final-sleep window base)×2(final-sleep window exponent). Number_of_Sleep_Connection IDs is the number of unicast Connection IDs (CIDs) corresponding to the power saving class.
(2) MOB_SLP-RSP
The BS transmits the MOB_SLP-RSP message to the MS to notify whether it approves or refuses the MS's transition to the sleep mode. Or the MOB_SLP-RSP message can be transmitted unsolicited. The MOB_SLP-RSP message includes parameters, i.e. IEs necessary for the MS to operate in the sleep mode, as illustrated in Table 2 below.
TABLE 2SizeSyntax(bits)NotesMOB_SLP-RSP_Message_format( ) { Management message type = 518 Number of Classes8Number of power saving classes for (i=0; i<Number_of_Classes; i++) {  Length of Data7  Sleep Approved1  Definition1  Operation1  Power_Saving_Class_ID6  if (Sleep Approved == 1) {   if (Operation = ) {    Start_frame_number6    Reserved2   }   if (Definition= 1) {    Power_Saving_Class_Type2    Direction2    initial-sleep window8    listening window8    final-sleep window base10     final-sleep window exponent3    TRF-IND required1    Traffic_triggered_wakening_flag1    Reserved1    if (TRF-IND required) {     SLPID10      Reserved2    }    Number_of_CIDs4    for (i=0; i<Number_of_CIDs; i++) {     CID16     }    if (MDHO or FBSS capability enabled) {If MDHO or FBSS capability isenabled in the REG-REQ/RSPmessage exchange.     Maintain Diversity Set and Anchor BS1     if (Maintain Diversity Set and Anchor BS) {      MDHO/FBSS duration(s)3     }    }   }   PaddingvariableIf needed for alignment to byteboundary   if (Operation = 1) {    Power Saving Class TLV encodedVariableinformation   }  } else {In case Sleep Approved == 0   REQ-duration8  }  TLV encoded informationvariable}
Referring to Table 2, the MOB_SLP-RSP message includes a plurality of IEs. The MOB_SLP-RSP message is also transmitted based on a basic CID of the MS.
Management Message Type indicates the type of the transmitted message. If Management Message Type is 51, it identifies the MOB_SLP-RSP message. Length of Data provides the number of bytes of a power saving class. Sleep Approved indicates whether the request for activation or deactivation of the power saving class is approved or not. If Sleep Approved is 1 and Operation is 1 (activation), Start_frame_number is included. If Sleep Approved is 1 and Definition is 1, Power_Saving_Class_Type, Direction, initial-sleep window, listening window, final-sleep window base, final-sleep window exponent, TRF-IND required, and TTWF are included. TRF-IND required applies only for power saving class Type 1. This means that the BS should transmit at least one MOB_TRF-IND message to the MS in every listening interval.
(3) MOB_TRF-IND
The MOB_TRF-IND message is transmitted from the BS to the MS during a listening interval, indicating the presence or absence of data for the MS. The MOB_TRF-IND message is broadcast or multicast, compared to the MOB_SLP-REQ or MOB_SLP-RSP message. The format of the MOB_TRF-IND message is given in Table 3 below.
TABLE 3SizeSyntax(bits)NotesMOB_TRF-IND_Message_format( ) { Management message type = 528 FMT1 if (FMT == 0) {  SLPID Group Indication bit-map32Nth bit of SLPID-Group indication bit-map [MSBcorresponds to N = 0] is allocated to SLPIDGroup that indicates MS with SLPID values fromN+32 to N+32+31Meaning of this bit0: There is no traffic for all the 32 MSs whichbelong to the SLPID-Group.1: There is traffic for at least one MS in SLPID-Group.  Traffic Indication BitmapvariableTraffic Indication bitmap comprises the multiplesof 32-bit long Traffic Indication unit.A Traffic Indication unit for 32 SLPIDs is addedto MOB_TRF-IND message whenever its SLPIDGroup is set to 1. 32 bits of Traffic IndicationUnit (starting from MSB) are allocated to MS inthe ascending order of their SLPID values:0: Negative indication1: Positive indication } else {  Num_Pos8Number of CIDs following  for (i=0; i<Num_Pos; i++) {   SLPIDs10  } } PaddingvariableIf needed, for alignment to byte boundary. TLV encoded ItemsVariable}
Referring to Table 3, the MOB_TRF-IND message indicates whether the BS has data to be transmitted to the MS. The MS receives the MOB_TRF-IND message during the listening interval and decides as to whether to transition to the awake mode or stay in the sleep mode.
If the MS is to transition to the awake mode, it checks frame synchronization. If a current frame sequence number is different from an expected frame sequence number, the MS may request retransmission of lost data. Meanwhile, if the MS fails to receive a MOB_TRF-IND message during the listening interval, or if the MS receives a MOB_TRF-IND message indicating the absence of data directed to the MS, the MS can be kept in the sleep mode.
The MOB_TRF-IND message also has a plurality of IEs. Management Message Type indicates the type of the transmitted message. If Management Message Type is 52, it identifies the MOB_TRF-IND message. FMT indicates the MOB_TRF-IND message takes the format of a SLeeP IDentifier (SLPID) bitmap or SLPIDs.
(4) DL Sleep Control Extended Subheader
The DL Sleep Control Extended Subheader is transmitted from the BS to the MS in order to activate or deactivate a power saving class. The DL Sleep Control Extended Subheader has the following configuration.
TABLE 4SizeName(bits)DescriptionPower_Saving_Class_ID6Power Saving Class ID thiscommand refers to.Operation11 = activate Power SavingClass0 = de-activate Power SavingClassFinal_Sleep_Window_Exponent3For Power Saving Class TypeIII only: assigned factor bywhich the final-sleep windowbase is multiplied in order tocalculate the duration ofsingle sleep windowrequested by the message.Final_Sleep_Window_Base10For Power Saving Class TypeIII only: the base for durationof single sleep windowrequested by the message.Reserved4
Referring to Table 4, the IEs of the DL Sleep Control Extended Subheader have been described with reference to the MOB_SLP-RSP message and thus their detailed description is not provided herein.
As described above, in the IEEE 802.16e communication system, for power saving class Type 1, the MS can maintain the sleep mode or deactivate it according to TTWF in the sleep-mode operation when receiving a MAC SDU from the BS during a listening interval. However, if the MS intends to maintain the sleep mode, the time of returning to the sleep mode after the listening interval is not clearly specified. In addition, TTWF is preset in initial MOB_SLP-REQ and MOB_SLP-RSP messages exchanged between the MS and the BS and it may occur that the TTWF needs to be changed during the sleep-mode operation. However, there is no specified operation for changing the TTWF.